This invention relates to high dynamic range image creation.
When taking photographs, cameras have limited dynamic range—that is, the photographer must choose the range of radiance values of interest and select the exposure time to optimally capture colors in that range. For example, a very short exposure time is required to capture the brightness of a sunlit portion of a scene, but may leave the other portions under-exposed. However, a longer exposure may saturate the film. To capture the full dynamic range in such a scene, a photographer can take a series of photographs with different exposures. These can then be combined into a single high dynamic range (“HDR”) image which includes the full range of radiance values found in the scene. An HDR image refers to an image with pixel values with floating point values having no specific upper value limit, unlike traditional digital images with pixel values limited by a particular bit depth.
Processes for creating HDR images require that all of the images of the scene input to the process are aligned pixel by pixel. For example, the contents of the image in the pixel at location (10, 52) in image 1 must align to the contents of the image 2 at location (10, 52) in image 2. When a camera is fixed, for example, by attaching it to a tripod, the resulting photographs can be expected to be perfectly aligned. However, photos may not be aligned if, for example, the camera is hand-held or otherwise not stationary. If a series of images are not aligned, computer methods can be used to align them prior to using them to create the HDR image, in a process called registration. Many processes exist for registration of multiple images.
Although these processes perform their best possible alignment of the input images, they may fail to align one or more of the images. In particular, it is often not possible to register an image that contains a moving object that is not visible in the other images. In any event, the HDR image created from the supposedly registered image will be partly distorted because some pixels will contain data from other points in the scene or from unwanted moving objects.